Semiconductor quantum dotdevice

ABSTRACT

A p-type semiconductor barrier layer is provided in the vicinity of undoped quantum dots, and holes in the p-type semiconductor barrier layer are injected in advance in the ground level of the valence band of the quantum dots. Lowering the threshold electron density of conduction electrons in the ground level of the conduction band of quantum dots in this way accelerates the relaxation process of electrons from an excited level to the ground level in the conduction band.

TECHNICAL FIELD

[0001] The present invention relates to electronic devices such as fieldeffect transistors and single-electron elements as well as tosemiconductor optical devices such as semiconductor lasers,semiconductor amplifiers, semiconductor optical switches, wavelengthconversion elements, and optical memories which are used in, forexample, optical communication and optical interconnections; and moreparticularly to a semiconductor quantum dot device which is providedwith quantum dots.

BACKGROUND ART

[0002] Quantum dots can be called boxes of quantized potential on thehyperfine scale in which carriers (conduction electrons and holes) areconfined in three dimensions. The density of the state function ofcarriers in a quantum dot is broken up according to a delta function,and the injected carriers are concentrated in these discrete energylevels. As a result, light which is emitted when the conductionelectrons and holes recombine in the interior of quantum dots is of highoptical intensity, and moreover, the width of the spectrum is extremelynarrow.

[0003] In a semiconductor laser in which quantum dots having theseproperties are provided in an active layer, the reduced thresholdcurrent which is required for laser oscillation and the decrease incarriers results in reduced internal loss, and therefore, improvedoscillation efficiency. In addition, the marked change in gain peak withrespect to change in the carrier density enables high-speed operation.(A semiconductor laser in which quantum dots are applied in the activelayer is hereinbelow referred to as a “quantum dot laser.”)

[0004] The realization of a quantum dot laser having these superiorproperties requires that quantum dots be formed at high density and withhigh uniformity, and moreover, without degrading crystallinity. Methodsthat are known from the prior art for forming such quantum dots includemethods which employ such techniques as lithography and dry etching.However, these methods have the drawback that processing damage whichoccurs in the semiconductor crystal as a result of lithography or dryetching greatly reduces the light emission efficiency.

[0005] As a method of eliminating this problem, a “Direct formationmethod (or Self-organizing method)” has been developed and put intoactual use in recent years. In this method, quantum dots are formedmerely by growing crystals.

[0006] The direct formation method was reported by D. Leonard et al. in1993. This method was proposed based on the finding that when alattice-misfit InGaAs is grown on a GaAs substrate, the InGaAs grows asa layer until it exceeds a critical film thickness, and upon exceedingthe critical film thickness, grows as islands, these island InGaAscrystals having a size of several tens of nanometers. This size issuitable for quantum dots [D. Leonard et al., Applied Physics Letters,Vol. 63, No. 23, pp. 3203-3205, December 1993]. The direct formationmethod was subsequently confirmed to be an excellent method for formingquantum dots.

[0007] The application of quantum dots formed by the direct formationmethod to semiconductor lasers is currently being widely studied. Forexample, G. T. Liu et al. have demonstrated that a quantum dot laserfabricated using the direct formation method operates with a lowerinjected current density than a prior-art semiconductor laser having anactive layer of a bulk structure or quantum well structure [G. T. Liu etal., Electronics Letters, Vol. 35, No. 14, pp. 1163-1165, Jul. 8, 1999].

[0008] Further, Chen has demonstrated that the operating current of aquantum dot laser which has been fabricated using the direct formationmethod is not influenced by temperature [H. Chen et al. ElectronicsLetters, Vol. 36, No. 20, pp. 1703-1704, Sep. 28, 2000].

[0009] However, a quantum dot laser which has an operating speedsurpassing that of a semiconductor laser of the prior art having anactive layer of bulk structure or quantum well structure has not yetbeen proposed. This is due to problems described hereinbelow that areinherent to quantum dots.

[0010] According to the Pauli exclusion principal, only two conductionelectrons and two holes can exist in each of the lowest energy levels(ground levels) of the conduction band and valence band in a quantum dotwhich has been quantized in three dimensions. As a result, if the groundlevels of the conduction band and valence band are already occupied byconduction electrons and holes (carriers), additional conductionelectrons or holes cannot be injected in the ground level. Thetransition of conduction electrons and holes from excited levels to theground levels of the conduction band and valence band is referred to as“relaxation.”

[0011] Accordingly, in a quantum dot laser which is oscillated by theinjection of a multiplicity of carriers into a plurality of quantum dotsformed in an active layer, the relaxation rate of electrons whichtransition from excited levels of the conduction band (an energy levelwhich is higher than the ground level of the conduction band) to theground level steadily drops with increase in the number of injectedconduction electrons. On the other hand, the relaxation rate of holeswhich transition from an excited level of the valence band (an energylevel which is lower than the ground level of the valence band) to theground level drops steadily with increase in the number of holes thatare injected.

[0012]FIG. 1 is a graph showing the emission (photoluminescence, PL)spectrum of a semiconductor sample in which a plurality of InAs quantumdots are formed on a GaAs substrate. The emission spectrum shown in FIG.1 shows the results of measuring light which is emitted when excitationlight is irradiated onto the sample to cause the generation of carriersinside the sample and the recombination of these generated carriersinside the InAs quantum dots. In addition, FIG. 2 is a graph showing therelation between the excitation power and the ratio (I₂/I₁) of emissionintensity I₁ from the ground level to emission intensity I₂ from theexcited level of the quantum dots for the sample that produced theemission spectrum of FIG. 1.

[0013] As can be understood from FIG. 1 and FIG. 2, as the excitationpower increases, emission intensity I₁ from the ground level firstincreases and gradually approaches saturation, following which emissionintensity I₂ from the excited level increases. The excited levelemission intensity I₂ begins to gradually increase before the emissionintensity I₁ from the ground level reaches complete saturation.

[0014] Here, if the thermal excitation of carriers to excited levels isconsidered, the change over time in the number of carriers at an excitedlevel can be given by the following equation (1): $\begin{matrix}{\frac{\partial N_{1}}{\partial t} = {\frac{N_{2}}{\tau_{r2}} - \frac{N_{1}}{\tau_{r2}} - \frac{N_{1}}{\frac{\tau_{2 - 1}}{\exp \left( {- \frac{\Delta \quad E_{2 - 1}}{kT}} \right)}}}} & (1)\end{matrix}$

[0015] In equation (1), N₁ and N₂ represent the number of carriers atthe ground level and excited level, respectively; τ_(r1) and τ_(r2)represent the radiative recombination times of the ground level andexcited level, respectively; τ₂₋₁ is the relaxation time of carriersfrom the excited level to the ground level; ΔE₂₋₁ represents the energydifference between the excited level and the ground level; k is theBoltzmann constant; and T is the absolute temperature.

[0016] In a stationary state, the left side of equation (1) is “0” andthe ratio (I₂/I₁) of excited level emission intensity I₂ to ground levelemission intensity I₁ can therefore be shown by equation (2) below:$\begin{matrix}{\frac{I_{2}}{I_{1}} = {\frac{\frac{N_{2}}{\tau_{r2}}}{\frac{N_{1}}{\tau_{r1}}} = \frac{\tau_{2 - 1} + {\tau_{r1}\quad {\exp \left( {- \frac{\Delta \quad E_{2 - 1}}{kT}} \right)}}}{\tau_{r2}}}} & (2)\end{matrix}$

[0017] As can be understood from equation (2), emission intensity ratio(I₂/I₁) is a function of the absolute temperature T.

[0018] In the above-described semiconductor sample (in which InAsquantum dots are formed on a GaAs substrate), if ΔE₂₋₁ is assumed to be73 meV, τ_(r1) to be 0.7 ns and τ_(r2) to be 0.5 ns at room temperature(293 K), the relation of the excitation power to relaxation time τ₂₋₁ ofcarriers, which is found from the change in the emission intensity ratio(I₂/I₁) shown in FIG. 2 is as shown in FIG. 3.

[0019] As can be understood from FIG. 3, as the excitation powerincreases, the relaxation time τ₂₋₁ of the carriers gradually increasesdue to filling of carriers at the ground level and thus approaches theradiative recombination times τ_(r1) and τ_(r2) (approximately 1 ns) ofthe ground level and excited level.

[0020] Thus, when the excitation power is 100 W/cm², i.e., when the samecarrier density can be obtained as during laser oscillation, the carrierrelaxation time τ₂₋₁ becomes 0.2 ns at room temperature (293 K). In thiscase, the 3-dB modulation bandwidth f_(3dB) given by the followingequation (3) is 0.8 GHz. In other words, the 3-dB modulation bandf_(3dB), which is the frequency range in which direct modulation ispossible, is limited to 0.8 GHz. $\begin{matrix}{f_{3{dB}} = \frac{1}{2\quad \pi \quad \tau_{2 - 1}}} & (3)\end{matrix}$

[0021] As is obvious from the foregoing explanation, the modulation ratein a quantum dot device such as a quantum dot laser drops due todecrease in the relaxation rate of carriers (increase in carrierrelaxation time τ₂₋₁), and this limited modulation rate complicates therealization of a modulation bandwidth on the order of 10 GHz, which isrequired in a current optical communication system.

[0022] The present invention was developed to overcome theabove-described problems inherent to the prior art and has as an objectthe provision of a quantum dot device which, by accelerating therelaxation of carriers to the ground level in quantum dots, can realizea broad modulation band of at least approximately 10 GHz and high-speedoperation.

DISCLOSURE OF THE INVENTION

[0023] The quantum dot device of the present invention is a constructionin which a p-type impurity is injected into quantum dots, whereby thisp-type impurity causes the generation of holes inside the quantum dots,these holes being at the ground level of the valence band of the quantumdots. In such a construction, the relaxation of carriers (carrierinjection) from an excited level to the ground level is accelerated, andincrease in the relaxation time of the carriers is therefore suppressed.As a result, the modulation band of the quantum dot device can beextended and a broad modulation band on the order of 10 GHz or more andan operating speed on the order of 10 GHz or higher can be realized.

[0024] In the present invention, the following relation is satisfied inthe above-described quantum dot device:

X₂≧X₁,

[0025] where X₁ (cm⁻²) is the surface density of the quantum dots and X₂(cm⁻²) is the surface density of the p-type impurity contained in thequantum dots.

[0026] As a result, relaxation process of electrons from the excitedlevel to the ground level is accelerated in the conduction band of thequantum dot.

[0027] Furthermore, if the following relation is satisfied:

X₂24 2×X₁,

[0028] the relaxation of carriers (carrier injection) to the groundlevel is remarkably accelerated since substantially all the groundlevels of the valence band of the quantum dots are occupied by holes.

[0029] In addition, when the absorption loss by the hole excessivelydoped to the ground level of the valence band is considered, it ispreferable to satisfy the following relation:

X₂≈2×X₁.

[0030] Further, the quantum dot device of the present invention is aconstruction in which a p-type impurity region is formed in the vicinityof a quantum dots, whereby holes generated in the p-type impurity regionare introduced into the quantum dots, these holes being at the groundlevel of the valence band of the quantum dots. Here, the “vicinity” ofthe quantum dot includes cases in which the p-type impurity region isformed in contact with the quantum dot and cases in which the p-typeimpurity region is formed separated by a distance within which theinjection efficiency of the holes does not drop excessively, such as adistance of 10 nm or less.

[0031] In such a construction as well, the relaxation of the carriers(carrier injection) from an excited level to the ground level isaccelerated, and increases in the relaxation time of the carriers cantherefore be suppressed. As a result, the modulation band of the quantumdot device can be extended and a broad modulation band on the order of10 GHz or more and an operating speed on the order of 10 GHz or highercan be realized.

[0032] In the present invention, the following relation is satisfied inthe above-described quantum dot device:

X₂>X₁,

[0033] where X₁ is the surface density of the quantum dots and X₂ is thesurface density of the p-type impurity contained in the p-type impurityregion.

[0034] As a result, relaxation process of electrons from the excitedlevel to the ground level is accelerated in the conduction band of thequantum dot.

[0035] Furthermore, if the following relation is satisfied:

X₂>2×X₁,

[0036] the relaxation of carriers (carrier injection) to the groundlevel is remarkably accelerated since substantially all the groundlevels of the valence band of the quantum dots are occupied by holes.

[0037] In addition, when the absorption loss by the hole excessivelydoped to the ground level of the valence band is considered, it ispreferable to satisfy the following relation:

X₂≈2×X₁.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a graph showing the emission photoluminescence spectrumof a semiconductor sample in which a plurality of InAs quantum dots havebeen formed on a GaAs substrate;

[0039]FIG. 2 is a graph showing the relation between the excitationpower and ratio (I₂/I₁) of emission intensity I₂ from the excited leveland emission intensity I₁ from the ground level of the quantum dots forthe sample in which the emission spectrum of FIG. 1 is obtained;

[0040]FIG. 3 is a graph showing the relation between the excitationpower and the relaxation time τ₂₋₁ of carriers from an excited level tothe ground level for the sample in which the emission spectrum of FIG. 1is obtained;

[0041]FIG. 4A is a sectional view showing the construction of the firstembodiment of the semiconductor quantum dot device of the presentinvention;

[0042]FIG. 4B is an energy band diagram of the semiconductor quantum dotdevice shown in FIG. 4A;

[0043]FIG. 5 is a graph showing the relation between the injectedcurrent and 3-dB modulation band f_(3dB) in a quantum dot laser havingan ordinary construction in which holes are not injected in advance intoquantum dots;

[0044]FIG. 6 is a graph showing the relation between the injectedcurrent and 3-dB modulation band f_(3dB) in a quantum dot laser havingthe construction of the present invention in which holes are injected inadvance into quantum dots;

[0045]FIG. 7A is a sectional view showing the construction of the secondembodiment of the semiconductor quantum dot device of the presentinvention;

[0046]FIG. 7B is an energy band diagram of the semiconductor quantum dotdevice shown in FIG. 7A;

[0047]FIG. 8 is a sectional view showing the construction of the firstexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied.

[0048]FIG. 9 is a sectional view showing the construction of the secondexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied;

[0049]FIG. 10 is a sectional view showing the construction of the thirdexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied;

[0050]FIG. 11 is a sectional view showing the construction of the fourthexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied; and

[0051]FIG. 12 is a sectional view showing the construction of the fifthexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied.

BEST MODE FOR CARRYING OUT THE INVENTION

[0052] The present invention is next described with reference to theaccompanying drawings.

[0053] [First Embodiment]

[0054] As shown in FIG. 4A, the semiconductor quantum dot deviceaccording to the first embodiment is a construction in which a pluralityof quantum dots 1 which have been formed from an undoped semiconductorare disposed on the surface of p-type semiconductor barrier layer 3 at asurface density of, for example, 5×10 cm⁻².

[0055] P-type impurity is injected into p-type semiconductor barrierlayer 3, whereby a multiplicity of holes 4 exist in the interior ofp-type semiconductor barrier layer 3. A portion of these holes 4 areintroduced into adjacent quantum dots 1. In other words, this is aconstruction in which quantum dots 1 shown in FIG. 4A contain holes 4even though these quantum dots 1 are not injected with p-type impurity(i.e., the quantum dots are undoped). As shown in FIG. 4B, these holes 4are in the ground level of the valence band of quantum dots 1, and theground level of the valence band is therefore filled by holes 4. Inaddition, quantum dots 1 are covered by undoped semiconductor barrierlayer 2 formed over quantum dots 1.

[0056] As described in the foregoing explanation, the ground level ofthe valence band of quantum dots 1 includes a number of states of two ifthe spin is taken into consideration. The amount of p-type impurityinjected into p-type semiconductor barrier layer 3 is therefore set togenerate holes at a surface density of 1×10¹¹ cm⁻² (=5×10¹⁰ cm⁻²×2).

[0057] In the present embodiment, holes 4 are introduced into quantumdots 1 in advance and the presence of these holes 4 in the ground levelsof the valence band of quantum dots 1 accelerates the process ofrelaxation of conduction electrons 5 from excited levels to the groundlevels of the conduction band of quantum dots 1. In other words, therelaxation time of conduction electrons 5 of quantum dots 1 is reduced.

[0058] It should be noted that the “radiative recombination of holes andconduction electrons” shown in FIG. 4B means the process in whichconduction electrons 5 transition from the ground level of theconduction band to the ground level of the valence band of quantum dots1, recombine with holes 4 which are at the ground level of the valenceband, and thus emit light.

[0059] The reason for the acceleration of the relaxation process ofconduction electrons 5 from an excited level to the ground level of theconduction band of quantum dots 1 is explained hereinbelow. Thefollowing explanation regards a case in which the quantum dot device ofthe present embodiment is applied to a semiconductor laser, but theexplanation also holds true when the quantum dot device of the presentinvention is applied to other optical devices.

[0060] Typically, when directly modulating a semiconductor laser, theinjected carriers, i.e., the injected current, are increased ordecreased in accordance with modulation signals while the semiconductorlaser is subjected to laser oscillation. Since the threshold value ofthe carrier density of the active layer (the threshold carrier density)is constant during laser oscillation, the intensity of emitted lightvaries in accordance with the increase and decrease of the carrier.

[0061] In the case of a quantum dot laser which includes a quantum dotstructure in the active layer, however, the threshold carrier density ofthe ground level of the quantum dots should be reduced to a minimum andthe relaxation process of the carrier to the ground level should beaccelerated in order to improve the operating (modulation) speed.

[0062] As shown in FIG. 4B, in quantum dot 1, conduction electrons 5 inthe conduction band are injected in the ground level of the conductionband, and holes 4 of the valence band are injected in the ground levelof the valence band. Since the effective mass of hole 4 is greater byone decimal place than the effective mass of conduction electron 5, theenergy difference between the ground level and the excited level of thevalence band is extremely small and holes 4 are easier to relax thanconduction electrons 5. In other words, holes 4, which have greatereffective mass, have properties which are easier to restrain in aquantum level such as the ground level or excited level than electrons,which have less effective mass. It can be seen that the difficulty ofrelaxing conduction electron 5, which have smaller effective mass, isthe governing factor which limits the high-speed modulation of thesemiconductor laser.

[0063] Therefore, the time necessary for the relaxation process ofconduction electrons 5 from an excited level to the ground level of theconduction band (i.e., the relaxation time) should be shortened as muchas possible to the. realize high-speed modulation characteristics of aquantum dot laser.

[0064] However, when the density of states (number of states) ofconduction electrons 5 or holes 4 in quantum dots 1 is approximatedusing a Gaussian function having an uneven spread ΔE from center energyE₀, gain g (the gain of only the ground level) of an active layer havinga quantum dot structure is as shown in equation (4): $\begin{matrix}{g = {\frac{h^{3}c^{2}N}{2\quad \pi \quad n^{2}E_{0}^{2}\tau_{sp}\Delta \quad E}{\sqrt{\frac{\ln \quad 2}{\pi}}\left\lbrack {f_{c} + \left( {1 - f_{v}} \right) - 1} \right\rbrack}}} & (4)\end{matrix}$

[0065] In equation (4), h is the Planck's constant, c is the speed oflight, N is the surface density of quantum dots 1, n is the index ofrefraction, τ_(sp) is the lifetime of spontaneous emission light, f_(c)is the occupation probability of conduction electrons 5 in theconduction band, and f_(v) is the occupation probability of conductionelectrons in the valence band. In addition, f_(c) and f_(v) are valuessuch that 0≦f_(c)≦1, 0≦f_(v)≦1.

[0066] When the gain g expressed by equation (4) exceeds the loss in aresonator, laser oscillation occurs in a quantum dot laser. When theloss of the resonator is maintained at a constant value by adopting thesame structure, causing the probability f_(v) that conduction electrons5 occupy the ground level of the valence band to approach “0”, that is,causing the probability that electrons are not present in the groundlevel of the valence band (the probability that holes 4 occupy theground level of the valence band) to approach “1” is effective forlowering the probability f_(c) that conduction electrons 5 occupy theground level of the conduction band during laser oscillation.

[0067] Thus, to realize this result, holes 4 can be introduced intoquantum dots 1 such that the ground level is occupied in advance byholes 4, whereby the probability f_(v) that conduction electrons 5occupy the ground level of the valence band of quantum dots 1 isdecreased.

[0068] Further, in order to set the probability f_(v) that conductionelectrons occupy the ground level of the valence band to “0”, the amountof holes 4 which are introduced into quantum dots 1 should be made equalto the number of states (density of states) of the ground level inquantum dots 1.

[0069] In a semiconductor laser of the prior art which uses a quantumwell structure or bulk structure as the active layer, however, it ispossible to lower the electron density during laser oscillation byappropriately setting the structure to inject holes into the activelayer.

[0070] In a construction in which the conduction band extendscontinuously from the ground level toward higher energy, as in asemiconductor laser of the prior art, lowering the threshold electrondensity narrows the spread of energy which is held by the conductionelectrons and narrows the spread of the gain energy, whereby theincrease in the gain peak due to increase in the electrons which areinjected becomes greater and the modulation speed of the laser outputbecomes higher. These points have already been reported by Uomi et al.[K. Uomi et al., Japanese Journal of Applied Physics, Vol. 29, No. 1,pp. 81-87, January 1990].

[0071] In the semiconductor laser provided with the quantum dots of thepresent embodiment, however, high-speed modulation is achieved byprinciples which are entirely different from those in the semiconductorlaser which uses the above-described bulk structure as the active layer.Specifically, the quantization of quantum dots 1 in allthree-dimensional directions results in discrete energy levels and anextremely narrow energy spread. As a result, the spread of the gainenergy is uniform regardless of the amount of injected carriers, and thepeak value of gain g of an active layer having a quantum dot structurevaries linearly with respect to the electron density.

[0072] According to the above-described concepts relating to asemiconductor laser which uses a quantum well structure or bulkstructure as the active layer, the modulation speed should be uniformand should not depend on the electron density. In the semiconductorlaser provided with the quantum dots of the present embodiment, however,the ground level of the conduction band is filled by conduction electron5, and the time (relaxation time) required for the relaxation process ofconduction electrons 5 which are in an excited level to the ground levelis therefore greater.

[0073] Research by the present inventor confirmed that this lengtheningof the relaxation time of conduction electrons 5 causes a drop in themodulation speed of a quantum dot laser. The introduction of holes 4 inadvance into the ground level of the valence band in quantum dots 1 andthe consequent reduction of the electron density of the ground level ofthe valence band of quantum dots 1 is effective for circumventing thisproblem.

[0074]FIG. 5 shows the relation between the injected current and 3-dBmodulation band f_(3dB) in a quantum dot laser having an ordinaryconstruction in which holes 4 are not introduced into quantum dots 1.This graph plots 3-dB modulation band f_(3dB) (a frequency range inwhich the response drops only 3 dB) for a case of fabricating a quantumdot laser having an ordinary construction in which holes 4 are notintroduced into quantum dots 1 and then directly modulating this quantumdot laser by a small signal and measuring the frequency responsecharacteristics. It should be noted that I_(th) is the oscillationthreshold current of this quantum dot laser.

[0075] As can be understood from FIG. 5, in a quantum dot laser in whichholes 4 are not introduced into quantum dots 1, although 3-dB modulationband f_(3dB) rises with increase in bias current I_(bias), 3-dBmodulation band f_(3dB) has an upper limit of approximately 2.5 GHz.

[0076] In contrast, FIG. 6 shows the relation between injected currentand 3-dB modulation band f_(3dB) in a semiconductor laser provided withthe quantum dots of the present embodiment in which holes 4 areintroduced to quantum dots in advance. The graph of FIG. 6 was obtainedby the same procedure as FIG. 5 with the exception of the introductionof holes 4 in quantum dots 1 in advance. In this quantum dot laser, theamount of doping of the p-type impurity in p-type semiconductor barrierlayer (p-type impurity region) 3 was set to ten times the number ofstates of the ground level of the valence band of quantum dots 1. Theground level of this valence band is therefore completely filled byholes 4. In addition, the semiconductor laser having the characteristicsof FIG. 6 is a construction having the same structure as the quantum dotlaser of ordinary structure which was fabricated for measuring thecharacteristics of FIG. 5, with the exception of the provision of p-typesemiconductor barrier layer (p-type impurity region) 3.

[0077] As can be clearly understood from FIG. 6, in the quantum dotlaser according to the present embodiment in which holes 4 have beeninjected in advance into quantum dots 1, the upper limit of 3-dBmodulation band f_(3dB) is 5 GHz, which is twice the level (2.5 GHz) ofthe quantum dot laser of ordinary structure, and the frequency band isextended to twice that of the quantum dot laser of ordinary structure.

[0078] However, in the quantum dot laser which was fabricated forobtaining the results of FIG. 6, the upper limit of 3-dB modulation bandf_(3dB) does not reach 10 GHz. This problem is thought to be aconsequence of setting the amount of doping of the p-type impurity inp-type semiconductor barrier layer (p-type impurity region) 3 to tentimes the number of states of the ground level of the valence band ofquantum dots 1, that is, doping which exceeds the amount of p-typeimpurity which is assumed to be necessary for filling the ground levelof the valence band with holes 4. Essentially, this problem arisesbecause excessive doping of holes 4 in the ground level of the valenceband of quantum dots 1 leads to absorption loss, and thus leads to aslight increase in the oscillation threshold carrier density.

[0079] The amount of doping of p-type impurity in p-type semiconductorbarrier layer (p-type impurity region) 3 is therefore preferablyregulated such that the amount of doping equals the number of states ofthe ground level of the valence band of quantum dots 1, that is, suchthat holes 4 fill all of the number of states which can be obtained inthe ground level of the valence band. In this way, a broad modulationband on the order of 10 GHz becomes possible and high-speed operation onthe order of 10 GHz or greater can be realized.

[0080] As described in the foregoing explanation, decreasing the densityof conduction electrons 5 of the ground level of quantum dots 1 in aquantum dot laser having the construction of the present embodimentenables an acceleration of the relaxation process of conductionelectrons 5 from an excited level to the ground level of the conductionband of quantum dots 1. For example, if the probability f_(c) thatconduction electrons 5 occupy the ground level of the conduction bandcan be lowered to approximately 0.1, the relaxation time τ₂₋₁ ofconduction electrons 5 can be made approximately 10 ps. According to theabove-described equation (2), it can be seen that 3-dB modulation bandf_(3dB) at this time surpasses 10 GHz.

[0081] [Second Embodiment]

[0082]FIG. 7A is a view showing the construction of the secondembodiment of the semiconductor quantum dot device according to thepresent invention, and FIG. 7B is an energy band diagram of this quantumdot device.

[0083] The quantum dot device of the second embodiment is a constructionin which quantum dots 1 are formed on undoped semiconductor barrierlayer 3 a, and p-type semiconductor barrier layer (p-type impurityregion) 2 a is formed on quantum dots 1. Holes 4 which are generated bythe p-type impurity introduced into p-type semiconductor barrier layer 2a are introduced into quantum dots 1. In the quantum dot deviceaccording to this second embodiment as well, action and effects can beobtained that are equivalent to the action and effects of the firstembodiment.

[0084] In addition, although examples were presented in above-describedthe first and second embodiments in which a p-type impurity is doped ina semiconductor barrier layer (p-type impurity region) which is disposedin the vicinity of quantum dots 1 and holes are introduced into thequantum dots from this semiconductor barrier layer, the holes may alsobe introduced into the quantum dots by directly doping the p-typeimpurity inside the quantum dots. According to experimentation by thepresent inventor, however, it was found that this type of compositionsuffers from the occurrence of flaws in the quantum dots and a drop inthe emission efficiency.

EXAMPLES

[0085] Explanation next regards examples of the present invention withreference to the accompanying drawing.

First Example

[0086]FIG. 8 shows the construction of a first example of a quantum dotlaser in which the semiconductor quantum dot device of the presentinvention has been applied.

[0087] The quantum dot laser shown in FIG. 8 was fabricated by thefollowing procedure which used an MBE (Molecular Beam Epitaxy)apparatus.

[0088] First, n-type AlGaAs cladding layer 22 (aluminum composition of0.3, thickness of 3 μm, and carrier concentration of 1×10¹⁸ cm⁻³),undoped GaAs optical confinement layer 23 (thickness of 0.15 μm), andundoped GaAs barrier layer 24 (thickness of 20 nm) are successivelygrown on n-type GaAs substrate 21.

[0089] The temperature of GaAs substrate 21 is next set to 490° C. andInAs is grown on undoped GaAs barrier layer 24 while regulating theamount such that the InAs having a thickness equivalent to a three-atomlayer is grown. At this time, undoped InAs quantum well layer (referredto as a “wefting layer”) 25 is first formed to a thickness equivalent toone- or two-atom layer, following which InAs forms islands which exceedthe critical film thickness of distortion while beryllium (Be), which isthe p-type impurity, is simultaneously supplied. In this way, aplurality of p-type InAs quantum dots 26 which contain p-type impuritywere formed on undoped InAs wefting layer 25.

[0090] The p-type InAs quantum dots which were thus obtained had aplanar disc shape and a surface density of 5×10¹⁰ cm⁻². In addition,each quantum dot had a diameter of 30 nm and a thickness (height) of 8nm.

[0091] In the first example, beryllium, which is the p-type impurity, issupplied together with the InAs when growing the InAs quantum dots asdescribed above, and the beryllium is doped inside the quantum dotswhile regulating the amount of this supply such that the surface densityof the beryllium (1×10¹¹ cm⁻²) at this time is twice the surface densityof quantum dots 1 (5×10¹⁰ cm⁻²). Holes are thus introduced as carriersinto each quantum dot, and these holes are at the ground level of thevalence band of each quantum dot.

[0092] Next, using the same MBE apparatus, undoped GaAs barrier layer 27(thickness of 20 nm), undoped GaAs optical confinement layer 28(thickness of 0.15 μm), p-type AlGaAs cladding layer 29 (aluminumcomposition of 0.3, thickness of 2 μm, and carrier concentration of5×10¹⁷ cm⁻³), and p-type AlGaAs cap layer 30 (aluminum composition of0.15, thickness of 0.5 μm, and carrier concentration of 5×10¹⁸ cm⁻³)were successively grown on the quantum dot structure to obtain thequantum dot laser shown in FIG. 8.

[0093] As shown in FIG. 8, p-type InAs quantum dots 26 in the firstexample are covered by undoped GaAs barrier layer 27. In addition,quantum dot structure 70 is formed by undoped InAs quantum well layer(wetting layer) 25 and a plurality of p-type InAs quantum dots 26 formedover this undoped InAs quantum well layer 25. Further, quantum dotstructure 70 and undoped GaAs barrier layers 24 and 27 which have beenformed above and below this quantum dot structure 70 constitute activelayer 71 of the quantum dot laser of the present example.

[0094] As previously described, in the quantum dot laser of the firstexample, beryllium (Be), which is a p-type impurity, is doped in thequantum dots to generate holes, and the ground level of the valence bandof each quantum dot is filled by these holes.

[0095] The ground level of the valence band of each quantum dot istherefore not occupied by conduction electrons during laser oscillation,whereby the density of conduction electrons in the quantum dots dropsand the relaxation process of conduction electrons from an excited levelto the ground level of the conduction band of the quantum dots isaccelerated.

[0096] In the first example, the relaxation rate of electrons wasaccelerated to approximately 10 ps. This means that the semiconductorlaser of the first example is capable of high-speed modulation of 10 GHzor more.

[0097] Although GaAs substrate 21 was used in the present example, anInP substrate may be used instead. In such a case, cladding layers 22and 29, optical confinement layers 23 and 28, and barrier layers 24 and27 may be formed of InAlGaAs, or may be formed of InGaAsP. In this typeof quantum dot laser, the emission wavelength from p-type InAs quantumdots 26 exceeds 1.3 μm, and this quantum dot laser is therefore wellsuited for use as a light source for use in long-wavelength opticalcommunication.

Second Example

[0098]FIG. 9 shows the construction of the second example of a quantumdot laser in which the semiconductor quantum dot device of the presentinvention is applied.

[0099] The quantum dot laser of the second example is a construction inwhich undoped InAs quantum dots 36, p-type GaAs barrier layer 37, andp-type GaAs optical confinement layer 38 are formed in place of p-typeInAs quantum dots 26, undoped GaAs barrier layer 27, and undoped GaAsoptical confinement layer 28, respectively, in the first example. Theconstruction is otherwise identical to that of the semiconductor laserof the first example, and constituent elements which are identical toelements in the first example are therefore identified by the samereference numbers and redundant detailed explanation is here omitted.

[0100] The quantum dot laser of the second example was fabricated by thefollowing process using an MBE apparatus as in the first example.

[0101] As in the first example, n-type AlGaAs cladding layer 22, undopedGaAs optical confinement layer 23, undoped GaAs barrier layer 24, andundoped InAs wetting layer 25 are successively grown on n-type GaAssubstrate 21.

[0102] A plurality of undoped InAs quantum dots 36 (diameter of 30 nm,thickness of 8 nm, and surface density of 5×10¹⁰ cm⁻²) are next formedon undoped InAs wetting layer 25. In the present example, in contrastwith the first example, beryllium (Be) is not supplied during growth ofInAs quantum dots 36.

[0103] In the present example, p-type GaAs barrier layer 37 (thicknessof 20 nm) and p-type GaAs optical confinement layer 38 (thickness of0.15 μm) are successively grown on undoped InAs quantum dots 36.

[0104] Finally, as in the first example, p-type AlGaAs cladding layer 29and p-type AlGaAs cap layer 30 were grown on p-type GaAs opticalconfinement layer 38 to obtain the quantum dot laser shown in FIG. 9.

[0105] As shown in FIG. 9, undoped InAs quantum dots 36 of the secondexample are covered by p-type GaAs barrier layer 37. In addition,quantum dot structure 70 a is formed by undoped InAs quantum well layer(wetting layer) 25 and the plurality of undoped InAs quantum dots 36formed on this undoped InAs quantum well layer 25. Furthermore, quantumdot structure 70 a and undoped GaAs barrier layers 24 and 27 formedabove and below this quantum dot structure 70 a constitute active layer71 a of the quantum dot laser of the present example.

[0106] Beryllium, which is the p-type impurity, is doped to aconcentration of 6×10¹⁵ cm⁻³ in p-type GaAs barrier layer 37 and p-typeGaAs optical confinement layer 38 which overlie undoped quantum dots 36,and these two layers therefore constitute p-type impurity regions.

[0107] In this construction, holes are generated with a surface densityof 1×10¹¹ cm⁻² in p-type GaAs barrier layer 37 and p-type GaAs opticalconfinement layer 38 and flow into quantum dots 36. At this time, twoholes are injected into each quantum dot, and these holes consequentlyfill the ground level of the valence band of each quantum dot.

[0108] As a result, in the quantum dot laser of the second example aswell, the electron density of the ground level of the conduction band ofquantum dots drops during laser oscillation, and the process ofrelaxation of conduction electrons from an excited level to the groundlevel of the conduction band is accelerated.

[0109] In the quantum dot laser of the second example, the relaxationtime of conduction electrons from an excited level to the ground levelof the conduction band of quantum dots can be accelerated up to theorder of 10 ps, and high-speed modulation of 10 GHz or more cantherefore be realized.

[0110] Although p-type impurity (Be) was directly doped into InAsquantum dots 26 in the first example, it was found that this type ofconstruction was problematic due to the occurrence of flaws in thequantum dots and the consequent drop in emission efficiency, asdescribed hereinabove. In the second example, rather than doping thep-type impurity (Be) into the quantum dots, the p-type impurity is dopedin semiconductor layers 37 and is 38 formed in the vicinity of quantumdots 36. Holes are then generated at a surface density of 1×10¹¹ cm⁻² inthese two semiconductor layers 37 and 38, and these holes are caused toflow into quantum dots 36.

[0111] Thus, the second example not only obtains the same effects as thefirst example but also allows an improvement in modulation speed whilesuppressing decrease in the light emission efficiency of the quantumdots.

Third Example

[0112]FIG. 10 shows the construction of the third example of a quantumdot laser in which the semiconductor quantum dot device of the presentinvention is applied.

[0113] The quantum dot laser of the third example is a construction inwhich p-type InAs wetting layer 45 is formed in place of undoped InAswetting layer 25 which was shown in the first example, and undoped InAsquantum dots 36 which were shown in the second example are formed inplace of p-type InAs quantum dots 26 which were shown in the firstexample. The construction is otherwise identical to the semiconductorlaser of the first example, and constituent elements which are the sameas elements in the first example are therefore identified by the samereference numerals and redundant detailed explanation is here omitted.

[0114] The quantum dot laser of the third example was fabricated by thefollowing procedure using an MBE apparatus as in the first example andsecond example.

[0115] First, as in the first example, n-type AlGaAs cladding layer 22,undoped GaAs optical confinement layer 23, and undoped GaAs barrierlayer 24 are first successively grown on n-type GaAs substrate 21.

[0116] InAs is then grown on undoped GaAs barrier layer 24 whilesupplying beryllium to form p-type InAs wetting layer 45 (thickness of0.3 nm). A plurality of undoped InAs quantum dots 36 are next formed onthis p-type InAs wetting layer 45 as in the second example, and undopedGaAs barrier layer 27 and undoped GaAs optical confinement layer 28 aresuccessively grown on these undoped InAs quantum dots 36.

[0117] Finally, as in the first example, p-type AlGaAs cladding layer 29and p-type AlGaAs cap layer 30 were successively formed on undoped GaAsoptical confinement layer 28 to obtain the semiconductor laser shown inFIG. 10.

[0118] As shown in FIG. 10, undoped InAs quantum dots 36 of the thirdexample are covered by undoped GaAs barrier layer 27. In addition,quantum dot structure 70 b is formed by p-type InAs quantum well layer(wetting layer) 45 and the plurality of undoped InAs quantum dots 36formed on this p-type InAs quantum well layer 45. Furthermore, quantumdot structure 70 b and undoped GaAs barrier layers 24 and 27 formedabove and below this quantum dot structure 70 b constitute active layer71 b of the quantum dot laser of the present example.

[0119] If the energy band structure is considered in the semiconductorlaser of the third example, the energy level of p-type InAs wettinglayer (quantum well layer) 45 is lower than the energy levels of undopedGaAs barrier layer 27 and undoped GaAs optical confinement layer 28. Inaddition, the energy level of undoped InAs quantum dots 36 is lower thanthe energy level of p-type InAs wetting layer 45. The nature ofconduction electrons is to flow from a region of high energy level to aregion of low energy level, and the conduction electrons therefore flowfrom GaAs layers 27 and 28 to wetting layer 45, and from wetting layer45 to quantum dots 36.

[0120] On the other hand, potential barriers against holes, thesepotential barriers being referred to as “spikes” or “notches,” occur asa result of the discontinuity of the energy band at the hetero-junctioninterface of GaAs barrier layer 27 and InAs wetting layer 45. Thesepotential barriers obstruct the movement of holes from GaAs barrierlayer 27 to InAs wetting layer 45.

[0121] InAs wetting layer 45 is therefore preferable to GaAs barrierlayer 27 as the p-type impurity region for injecting holes into InAsquantum dots 36 due to the more efficient injection of holes into InAsquantum dots 36.

[0122] In the quantum dot laser of the third example, p-type impurity isdoped in InAs wetting layer 45, and GaAs barrier layer 27 is undoped. Asa result, not only can the same effects as the first example berealized, but holes can also be efficiently introduced into InAs quantumdots 36 from p-type InAs wetting layer 45 without any obstruction of thetransition of conduction electrons.

Fourth Example

[0123]FIG. 11 shows the construction of the fourth example of thequantum dot laser in which the semiconductor quantum dot device of thepresent invention is applied.

[0124] The fourth example of the quantum dot laser is a construction inwhich undoped InAs quantum dots 36 which were shown in the secondexample are formed in place of p-type InAs quantum dots 26 which wereshown in the first example, and in which p-type GaAs barrier layer 51and undoped AlGaAs buried layer 52 are formed in place of undoped GaAsbarrier layer 27 which was shown in the first example. The constructionis otherwise the same as that of the semiconductor laser of the firstexample, and constituent elements which are identical to those of thefirst example are therefore identified by the same reference numeralsand redundant detailed explanation is here omitted.

[0125] The semiconductor quantum dot laser of the present example wasfabricated by the following procedure which uses an MBE apparatus.

[0126] First, as in the first example, n-type AlGaAs cladding layer 22,undoped GaAs optical confinement layer 23, undoped GaAs barrier layer24, and undoped InAs wetting layer 25 were successively grown on n-typeGaAs substrate 21.

[0127] Next, as in the second example, a plurality of undoped InAsquantum dots 36 were formed on undoped InAs wetting layer 25, andfurther, undoped AlGaAs buried layer 52 (thickness of 8 nm) wasselectively formed on undoped InAs wetting layer 25 so as to bury thegaps between the quantum dots. At this time, the thickness of undopedAlGaAs buried layer 52 was equal to the height of quantum dots (8 nm).

[0128] Continuing, p-type GaAs barrier layer 51 (thickness of 20 nm, andcarrier concentration of 5×10¹⁶ cm⁻³) was grown on the quantum dots andundoped AlGaAs buried layer 52, following which undoped GaAs opticalconfinement layer 28, p-type AlGaAs cladding layer 29, and p-type AlGaAscap layer 30 were successively grown on this p-type GaAs barrier layer51 as in the first example, whereby the semiconductor laser shown inFIG. 11 was obtained.

[0129] As shown in FIG. 11, undoped InAs quantum dots 36 of the fourthexample are entirely covered by undoped AlGaAs buried layer 52 with theexception of the upper surfaces of undoped InAs quantum dots 36, and theupper surfaces of undoped InAs quantum dots 36 are covered by p-typeGaAs barrier layer 51. Quantum dot structure 70 a is formed by undopedInAs quantum well layer (wetting layer) 25 and the plurality of undopedInAs quantum dots 36 formed on this undoped InAs quantum well layer 25.Furthermore, quantum dot structure 70 a together with underlying undopedGaAs barrier layer 24 and overlying p-type GaAs barrier layer 51, andundoped AlGaAs buried layer 52 constitute active layer 71 c of thequantum dot laser of the present example.

[0130] In the quantum dot laser of the fourth example, AlGaAs buriedlayer 52, which has an energy level which is higher than GaAs, is buriedat the side surfaces of undoped InAs quantum dots 36 as described in theforegoing explanation, and p-type GaAs barrier layer 51 is formed incontact with the upper surfaces of quantum dots 36.As a result, only theupper surfaces of InAs quantum dots 36 contact p-type GaAs barrier layer51.

[0131] The flow path of carriers to undoped quantum dots 36 of quantumdot laser of the fourth example is next considered.

[0132] Carriers flow into undoped quantum dots 36 from below quantumdots 36 by way of undoped InAs wetting layer 25. The formation ofundoped AlGaAs buried layer 52 at the side surfaces of quantum dots 36prevents the flow of carriers into quantum dots 36 from these sidesurfaces. On the other hand, carrier flows in from above quantum dots 36from p-type GaAs barrier layer 51 (which has a smaller band gap thanAlGaAs buried layer 52) which contacts the upper surfaces of quantumdots 36. As a result, holes are introduced into quantum dots 36 directlyfrom p-type GaAs barrier layer 51 and enter the ground level of thevalence band.

[0133] In the semiconductor laser of the fourth example, conductionelectrons injected from the outside enter undoped quantum dots 36 by wayof undoped InAs wetting layer 25 during operation (during laseroscillation) and consequently undergo radiative recombination insidequantum dots 36 with holes which have been injected from outside thesemiconductor laser. The conduction electrons injected from the outsidedo not recombine inside wetting layer 25 with holes which have beenintroduced in advance.

[0134] As a result, the semiconductor quantum dot laser of the presentexample not only provides the same effects as the first example but canalso raise the efficiency of radiative recombination in quantum dots 36.

Fifth Example

[0135]FIG. 12 shows the construction of an optical amplifier (quantumdot optical amplifier) in which the semiconductor quantum dot device ofthe present invention is applied.

[0136] The semiconductor optical amplifier of the present example is aconstruction in which a semiconductor layer is formed which has alaminated structure identical to the quantum dot laser of the secondexample shown in FIG. 9, and in which low-reflection films 61 a and 61 bare applied to and formed at both end surfaces of this semiconductorlayer. The construction of the semiconductor layer is equivalent to thatof the second example and redundant explanation is therefore hereomitted.

[0137] As shown in FIG. 12, in the semiconductor optical amplifier ofthe present example, signal light introduced into the semiconductoroptical amplifier by low-reflection films 61 a and 61 b which are formedat both end surfaces of the laminated structure (optical waveguide) isamplified by stimulated emission in active layer 71 a having quantum dotstructure 70 a.

[0138] Gain in the ground level of quantum dots 36 which is lost bystimulated emission is recovered by the relaxation of conductionelectrons and holes to the ground levels of the conduction band andvalence band of quantum dots 36. Thus, as in the quantum dot laser ofthe second example, the injection of holes in advance in the groundlevel of the valence band of quantum dots 36 enables an acceleration ofthe process of relaxation of conduction electrons. As a result, opticalamplification can be carried out that follows signal light whichundergoes high-speed modulation in excess of 10 GHz.

[0139] [Modifications]

[0140] Although examples have been described in the above-describedfirst to fourth examples in which the semiconductor quantum dot deviceof the present invention was applied to a semiconductor laser, thepresent invention is not limited to this form and can be applied to anyother optical device that uses quantum dots.

[0141] Further, although an example was described in which thesemiconductor quantum dot device of the present invention was applied toa semiconductor optical amplifier in the fifth example, the presentinvention is not limited to this form and can be applied in otheroptical devices such as semiconductor optical switches for implementingsignal light switching and semiconductor wavelength conversion elementsfor converting the wavelength of signal light. Accordingly, the presentinvention can be applied to high-speed switching and wavelengthconversion of signal light by the same construction and principles asthe semiconductor optical amplifier.

[0142] Further, as devices that use quantum dots, applications are alsopossible to field effect transistors, single-electron control memoies,or optical memories.

[0143] Although examples were described in the above-described first tofifth examples in which InAs was used for the wetting layer and quantumdots, the present invention is not limited to this form, and the wettinglayer and quantum dots may employ semiconductors other than InAs. Forexample, InGaAs, GaInNAs, InGaP, and InGaN may also be used as thematerial that makes up the wetting layer and quantum dots.

What is claimed is:
 1. A semiconductor quantum dot device that isprovided with a plurality of quantum dots, comprising: a p-type impurityarranged inside said quantum dots for generating holes, said holesfilling a ground level of a valence band of said quantum dots.
 2. Thesemiconductor quantum dot device according to claim 1, whereinconcentration of said p-type impurity is a value which generates holesin said quantum dots in a number equal to or greater than a number ofstates which can exist in the ground level of the valence band of saidquantum dots.
 3. The quantum dot device according to claim 2, whereinwhen X₁ is a surface density of said quantum dots and X₂ is a surfacedensity of the p-type impurity which is contained in said quantum dots,X₂≧X₁.
 4. The quantum dot device according to claim 3, wherein when X₁is the surface density of said quantum dots and X₂ is the surfacedensity of the p-type impurity which is contained in said quantum dots,X₂≧2×X₂.
 5. A semiconductor quantum dot device provided with a pluralityof quantum dots, comprising: a p-type impurity region disposed adjacentto said quantum dots, a p-type impurity being injected into said p-typeimpurity region for generating holes for filling a ground level of avalence band of said quantum dots.
 6. The semiconductor quantum dotdevice according to claim 3, wherein the concentration of said p-typeimpurity is a value for introducing, into said quantum dots, holes of anumber which is equal to or greater than a number of states which canexist in the ground level of the valence band of said quantum dots. 7.The quantum dot device according to claim 6, wherein when X₁ is asurface density of said quantum dots and X₂ is a surface density ofp-type impurity contained in said p-type impurity region, X₂≧X_(1.) 8.The quantum dot device according to claim 7, wherein when X₁ is thesurface density of said quantum dots and X₂ is the surface density ofthe p-type impurity contained in said p-type impurity region, X₂≧2×X₂.9. The quantum dot device according to claim 5, comprising: a p-typesemiconductor cladding layer and an n-type semiconductor cladding layerwhich are arranged with said quantum dot interposed; wherein said p-typeimpurity region is formed between said quantum dots and said p-typesemiconductor cladding layer.
 10. The quantum dot device according toclaim 5, wherein said p-type impurity region is formed on a p-typesemiconductor wetting layer which is adjacent to said quantum dots. 11.The quantum dot device according to claim 5, wherein said p-typeimpurity region is formed in contact with ends of said quantum dots andat a position which confronts a p-type semiconductor wetting layer whichis adjacent to said quantum dots with said quantum dot interposed. 12.The quantum dot device according to claim 11, comprising: asemiconductor buried layer which buries gaps between said quantum dots;wherein a band gap of said p-type impurity region is smaller than a bandgap of said semiconductor buried layer.